Well, I have one advice to you: furnish it as you would furnished any room from the main floor.
Furnish the space with bright and fresh items, you will quickly forget that you are in a basement!
I found examples of some amazing basement rooms, tastefully decorated:
I would recommend to paint the walls the same colour as the ceilings or at least in a very close shade. It will make the ceilings appear higher.
Incorporate luxurious details like the fireplace in the example below: